Project Summary Recently we discovered that cholesterol metabolically reprograms tumor-infiltrating T cells so that they become exhausted. Our unpublished, preliminary studies showed that tumor tissues have a much higher cholesterol content compared with normal tissues, and the PD-1high2B4high CD8+ T cells in tumor-infiltrating T cells have significantly higher cholesterol content than PD-1med2B4med cells, which in turn have significantly higher cholesterol content than PD-1low2B4low cells in different murine tumor models. The same was observed in human multiple myeloma and colon tumor samples of. We also showed that the PD-1high2B4high CD8+ T cells have significantly higher LAG-3 and TIM-3 (other T-cell exhaustion markers) expression than PD-1med2B4med cells, and the PD-1med2B4med cells have significantly higher LAG-3 and TIM-3 expression than PD-1low2B4low cells. Consistently, sorted PD-1high2B4high CD8+ T cells displayed much weaker cytolytic activity against target tumor cells than PD-1med2B4med CD8+ T cells. Adding cholesterol to the culture of tumor-specific CD8+ T cells upregulated their expression of PD-1 and other exhaustion markers and reduced their cytolytic activity. Conversely, reducing cholesterol content in sorted PD-1high2B4high tumor-infiltrating CD8+ T cells downregulated their expression of PD-1 and other exhaustion markers and enhanced their cytolytic activity. Based on these novel findings, we hypothe size that the tumor and its microenvironment induce effector T -cell exhaustion by using cholesterol to metabolically reprogram and upregulate the expression of immune inhibitory receptors and exhaustion markers on CD8+ cells. Aim 1 will determine the mechanisms underlying cholesterol-induced CD8+ T-cell exhaustion, and Aim 2 will reprogram CD8+ T-cell metabolism and/or the tumor microenvironment to enhance the antitumor effects of tumor-specific CD8+ T cells. Completing this project will give us in-depth understanding of the mechanisms involved in how tumor -derived cholesterol metabolically repr ograms tumor- infiltrating T cells so that they become exhausted. Understanding the mechanisms will allow us and others to identify novel therapeutic targets and develop new methods to improve the efficacy of T cell- or immune checkpoint blockade-based immunotherapy in cancer.